Listen to hip-hop, rap and, if you can find it, beat poetry. Hip-hop has a certain rhythm and beat to it. Listen to the beat and pay attention to how it matches with the lyrics of the song. Getting a feel for the music is important, especially since freestylers will have maybe three or four repetitions of the beat before they have to start laying down rhymes. If you can set a poem to music, then you are essentially creating rap right there.
Practice rhyming and writing poetry. You can sit down in a study and create rhyming combinations as fast as you can, or you can play word games with your friends. Basically you need to train your mind to think of one word and then think of words that rhyme. For instance, if you think of the word "wood," then you might list out "good," "hood," and "should."
Practice freestyling. Once you've gotten your brain into creating rhymes, you need to start creating entire lines on the fly. For instance, using the word "good" you might create a first line such as, "I'm an outlaw baby, only bad when I'm good," promptly followed by the line, "Running eight horses loud through my hood." The basic concept of freestyling isn't hard, but to get your brain and mouth to work in concert, you have to do it often and repetitively to really make your own rhymes.